1. Technical Field
The present invention relates to frequency-adjustable antenna devices, and particularly to an antenna device and a wireless communication apparatus for multisystem communication.
2. Background Art
Examples of known techniques relating to antenna devices of this type are described in Patent Document 1 and Patent Document 2.
Patent Document 1 describes a frequency variable antenna having a loop-shaped radiation electrode that performs a monopole antenna operation. The frequency variable antenna is provided with a frequency variable circuit in the middle of the radiation electrode. Thus, by externally applying a voltage to the frequency variable circuit to vary a reactance component of the frequency variable circuit, it is possible to vary the frequency while maintaining good gain.
Patent Document 2 describes an antenna device having an antenna main body and a variable capacitance diode that forms a resonant circuit at a base of the antenna main body. By applying a tuning voltage to vary the electrostatic capacitance of the variable capacitance diode, a desired frequency can be obtained.
Recently, as mobile phones have become multifunctional, it has become necessary to mount various systems of different frequencies on the same substrate. To realize such a multifunctional mobile phone, it is necessary to mount many antenna units corresponding to various systems close together in a small antenna mounting area.
However, when a plurality of antenna units are mounted, if antenna units having close fundamental frequencies are located close together or if a first antenna unit and a second antenna unit having a fundamental frequency close to a harmonic frequency in the first antenna unit are located close together, interference may occur and cause degradation in characteristics of these antenna units.
Moreover, because of enhanced multifunctionality of mobile phones, since a substrate is mostly occupied by functional circuits other than radiation electrodes of antenna units, a mounting area for mounting the radiation electrodes is reduced. At the same time, as the size of mobile phones shrinks, a mounting area for mounting radiation electrodes becomes extremely small.
Thus, under circumstances where it is necessary to mount radiation electrodes of antenna units for various systems in a very small area, antenna units having close frequencies need to be arranged close together.
Therefore, it is hoped that there will be developed an antenna device in which no interference occurs even if many antenna units corresponding to various systems are mounted close together in a small area.    Patent Document 1: PCT International Publication No. WO2004/109850    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-232313
However, with the foregoing known techniques, it is difficult to meet the expectation described above.
Specifically, in the frequency variable antenna described in Patent Document 1, a current density in a radiation electrode extending from a power feeder to a frequency variable circuit is very high. Therefore, if a number of such antennas are arranged close together, currents flowing through bases of antennas having close fundamental frequencies may cause very strong coupling of magnetic fields, and interference may occur between these antennas. This may result in deterioration in isolation between antennas and degradation of antenna gain.
Additionally, since components are mounted on a surface of a substrate to form a frequency variable circuit, these components protrude from the surface of the substrate by the thicknesses of these components. This not only hinders a size reduction in the direction of thickness of the mobile phone, but also causes a problem of strength of component mounting.
In the antenna device described in Patent Document 2, current densities of both fundamental waves and harmonics are very high at the base of the antenna main body. Therefore, by increasing the inductance of a matching circuit at the base of the antenna main body, fundamental waves and harmonics can be changed simultaneously. However, if a fundamental frequency is changed, the corresponding harmonic frequency is changed by an amount as much as several times greater than the amount of change of the fundamental frequency. Thus, since fundamental waves and harmonics cannot be independently controlled, the harmonic frequency may overlap with the fundamental frequency of another system, and thus, mutual interference may occur.
Thus, with the techniques described in Patent Documents 1 and 2, it is difficult to simultaneously solve the problem of interference between fundamental frequencies and the problem of interference between a harmonic frequency and a fundamental frequency. Even if a number of these antenna devices are mounted, it is not possible to meet the expectation described above.
Moreover, in the techniques described above, when many antenna units are put together in one place, radiation electrodes and the like of the respective antenna units are disposed on different substrates. This means that costs involved in forming a radiation electrode and the like on each substrate are multiplied by the number of antenna units. Additionally, when antenna units individually designed are integrated into one place, since their characteristics may be changed depending on the installation conditions, each antenna unit needs to be adjusted in response to the changes in characteristics. This makes the process more complicated.